Most commercial processes for the production of fuels and chemicals are designed to be continuous because continuity achieves high efficiency and high utilization of expensive capital intensive equipment. It is also desirable for fuel and chemical production processes to use solar energy as the source of heat because solar energy is both plentiful and inexpensive. However, the sun does not supply either a continuous or a uniform amount of energy. Energy levels are reduced or eliminated as a result of cloud cover and normal solar diurnal variation caused by the earth's rotation. Therefore, for solar energy to be effectively used for the production of fuels or chemicals, commercial processes utilizing solar energy must compensate for the intermittent reduction in energy.
There are a variety of ways of compensating for intermittent energy reduction. A heat transfer fluid may be stored at high temperatures and then used during nighttime or cloud cover to maintain the required conditions. Unfortunately, if the stored high temperature fluid is exhausted before the clouds disperse or during the night, the plant will shut down.
To permit more extended nighttime operation, hybrid systems consisting of a solar reactor and a fossil-fuel-fired reactor have been recommended. Such hybrid systems eliminate or reduce the need for heat-transfer fluid storage because the operation of both the solar reactor and the fossil-fuel-fired reactor is modulated to produce a constant net heat output. Thus, during times of peak solar energy, all heat is supplied by the sun. In contrast, at night, the fossil-fueled reactor operates at full load and the solar reactor shuts down. In this type of system, however, solar energy provides only about 30% of the energy requirements.